British Prime Minister Theresa May has said the Russian state was culpable in the nerve agent attack in Salisbury on former Russian double agent Sergei Skripal and his daughter Yulia and the United Kingdom is expelling 23 diplomats.

"All who [have] been identified as undeclared intelligence officers. They have just one week to leave," she said, Sky News reports.

"This will be the single biggest expulsion for over 30 years and it will reflect the fact that this is not the first time the Russian state has acted against our country."

She said Russia's response "has shown complete disdain" and the country has offered no explanation for the Russian-made Novichok nerve agent used in the attack.

The PM said the matter had been treated with "sarcasm, contempt and defiance."

"It must be met with a full and robust response," she said.

"We've agreed immediate actions to dismantle the Russian espionage network in the UK."

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She said no ministers or members of the Royal Family will be attending this summer's World Cup.

At the same time, the Russian ambassador warns Britain to expect retaliation for diplomat expulsions.

Alexander Vladimirovich Yakovenko tells Sky's Senior Political Correspondent Jason Farrell the UK's actions are "unacceptable" and that Moscow considers the expulsion of Russian diplomats "a provocation."

He says the measures have "nothing to do with the situation that we have in Salisbury."

"This is a really serious provocation."